1. Field of the Invention
The present invention relates to a hydraulic shock absorber suspended between a wheel side which is a first portion of a shock absorbing body and a vehicle body side which is a second portion of the shock absorbing body, for absorbing an impact force supplied to the shock absorbing body in a case in which the shock absorbing body is, for example, included in a suspension system of an automobile.
2. Description of the Related Art
One conventional hydraulic shock absorber is disclosed in JP-A-2004-232845. According to the disclosed publication, the hydraulic shock absorber is a single-rod type including a cylinder tube connected to a first portion of a shock absorbing body which is a suspension system of an automobile; a free piston slidably fitted in the cylinder tube in the axial direction for dividing the cylinder tube into an oil chamber filled with hydraulic oil and a gas chamber filled with high-pressure gas; a piston slidably fitted in the oil chamber in the axial direction for dividing the oil chamber into first and second oil chambers; a piston rod projecting from the piston and extending to the outside of the cylinder tube through the first oil chamber located opposite to the gas chamber with its extension end being connected to a second portion of the shock absorbing body; and a damping force generator for generating a damping force with the hydraulic oil flowing between the first and second oil chambers.
In the above case, the gas chamber and the gas filled in the gas chamber function as an oil chamber pressurizing device for pressurizing the hydraulic oil in the first and second oil chambers. With the oil chamber pressurizing device, the occurrence of bubbles generated in the hydraulic oil due to the negative pressure in the first and second oil chambers when the shock absorber is operated or the volume of the hydraulic oil varies with a change in temperature can be prevented. Thereby, damping characteristics of the shock absorber can be better maintained.
A shock absorber used in a suspension system of an automobile is generally configured as follows. The cylinder tube is connected to the wheel side and the extension end of the piston rod is connected to the vehicle body side via a rubber mount. A suspension spring is provided between the cylinder tube and the vehicle body side for extending the shock absorber.
Each of the axial end surfaces of the piston is a pressure receiving surface which receives pressure from the hydraulic oil. In the single-rod type shock absorber described above, the pressure receiving area of one pressure receiving surface is larger than that of another pressure receiving surface from which the piston rod projects by the sectional area of the piston rod. Therefore, as described above, when the hydraulic oil is pressurized by the pressure of the gas of the oil chamber pressurizing device, the piston is urged in an extending direction of the piston rod by an urging force (R), which is a pressurized reaction force generated by the pressure of the hydraulic oil based on the oil chamber pressurizing device and the difference of the pressure receiving area described above. As a result, the single-rod type shock absorber has characteristics that normally cause the extension of the shock absorber in the free state.
Therefore, the vehicle body side weight (W) corresponding to the shock absorber is supported on the wheel side by the spring reaction force (F) of the suspension spring and is also supported on the wheel side by the urging force (R) of the oil chamber pressurizing device via the piston, the piston rod, and the rubber mount on the shock absorber (W=F+R). In this case, the urging force (R) urges the rubber mount upward. Therefore, for example, when the automobile is in a stationary state, an initial elastic deformation appears on the rubber mount in the upward extending direction caused by the urging force (R).
When the shock absorber makes a compression or extension movement with an impact force under conditions described above while the automobile is driven, it is preferable that the rubber mount elastically deforms with an elasticity as uniform as much as possible (desired elastic deformation) during each of the compression or extension movement.
If the rubber mount is a single element not provided with the shock absorber, the rubber mount is designed to have rubber mount characteristics such that when an upward load is applied, the rubber mount softly elastically deforms in the extension direction, while when a downward load is provided, the rubber mount stiffly elastically deforms in the compression direction. Thus, a lot of effort is made so that the rubber mount with the desired elastic deformation is obtained in each of the compression or extension movement of the shock absorber described above under the condition that there is an initial elastic deformation on the rubber mount caused by the urging force (R) in the stationary state of the automobile.
Meanwhile, another conventional hydraulic shock absorber is disclosed in JP-A-Hei11-165521. According to this publication, the hydraulic shock absorber is a double-rod type including a cylinder tube connected to a first portion of a shock absorbing body; a piston slidably fitted in the cylinder tube in an axial direction thereof for dividing the cylinder tube into first and second oil chambers; a main piston rod extending from the piston to the outside of the first oil chamber through the first oil chamber with its extension end being connected to a second portion of the shock absorbing body; a sub piston rod extending from the piston to the outside of the second oil chamber through the second oil chamber; an oil chamber pressurizing device for pressurizing the hydraulic oil in the first and second oil chambers; and a damping force generator for generating a damping force with the hydraulic oil flowing between the first and second oil chambers.
Recently, in a single-rod type shock absorber used in a suspension system of an automobile as described in JP-A-2004-232845, there is a case in which the diameter of the piston rod in the shock absorber is required to have an increased capacity and strength. When the diameter of the piston rod is increased, the urging force (R) increases in a rate corresponding to the increased area of the piston rod.
However, even though the urging force (R) in the shock absorber increases, it is not easy to obtain the desired rubber mount characteristics for achieving the desired elastic deformation on the rubber mount due to the limitations of the construction of the rubber mount and the limitations of its strength. Therefore, under the above situation, it is very difficult to practically achieve the desired elastic deformation.
Meanwhile, there may be a case in which a shock absorber is changed to a double-rod type as described in JP-A-Hei 11-165521, or a single-rod type shock absorber is used in a suspension system of an automobile described in JP-A 2004-232845 with the rubber mount unchanged.
However, in the pistons of the double-rod type shock absorber, each piston rod extends from each axial end surface to the outside of each cylinder tube. Accordingly, the pressure receiving area of each pressure receiving surface of the piston becomes generally equal. Therefore, though the oil chamber pressurizing device pressurizes the hydraulic oil, the urging force (R), which is a pressurized reaction force of the gas, is not generated. As a result, the piston is not urged toward either of the axial directions together with each piston rod. That is, the double-rod type shock absorber does not have characteristics that make the extension movement in the free state.
Therefore, as described above, when a double-rod type shock absorber is provided in a suspension system of an automobile to which a single-rod type shock absorber should be used, the vehicle-body side weight (W) is supported on the wheel side only by the spring reaction force (F) by the suspension spring and no urging force (R) of the single-rod type shock absorber is applied to the rubber mount. Therefore, the rubber mount stays in the free state and the initial elastic deformation generated by using the single-rod type shock absorber does not occur.
As a result, for example, when an impact force is applied to the shock absorber in its axial direction thereby making the compression movement while the automobile is running on a flat road, the rubber mount first starts to deform elastically from the free state, and the elastic deformation proceeds to reach the initial elastic deformation state. Then the rubber mount exhibits damping characteristics similar to those of the single-rod type shock absorber for the first time.
Therefore, in a case in which the double-rod type shock absorber is used instead of the single-rod type shock absorber with the rubber mount unchanged, a time lag occurs in generating a damping force, which makes the rubber mount characteristics unsuitable. This may cause deterioration of the riding comfort of the automobile, and therefore it is not preferred.